The long-term goal of our laboratory is to develop a biologically relevant mouse model of endometrial carcinoma for the purpose of addressing clinically important questions. Endometrial carcinoma is the most common malignancy of the female genital tract in the United States, and uterine endometrioid carcinoma (UEC) is the most prevalent subtype. UEC arises from proliferative endometrium, in the setting of unopposed estrogen, via a continuum of histopathological precursor lesions called hyperplasias. The direct precursor of UEC, complex atypical hyperplasia (CAH), closely resembles UEC with the exception that it lacks stromal invasion. Because of the inability to predict which precursor lesions may progress and the morphologic ambiguities of distinguishing between CAH and UEC on endometrial sampling, numerous women undergo hysterectomy for benign, non-invasive disease. Thus, a more thorough understanding of the differences between CAH and UEC, and the role of both hormonal and genetic factors on the development and progression of endometrial tumorigenesis would have a substantial impact on the diagnosis and management of women with proliferative endometrial lesions. The two most common molecular genetic abnormalities yet identified in UEC are mutations in the PTEN tumor suppressor gene and microsatellite instability (MI) which are present in 30-50% and 20% of tumors, respectively. PTEN mutations and MI have also been detected in a subset of CAH suggesting that both alterations occur relatively early in the pathogenesis of UEC. Recently it has been reported that CAH develops in 100% of female Pten mice and progresses to carcinoma in approximately 20% of mice at 40 weeks of age. In this proposal we will further develop and exploit this model through the following specific aims: 1. To identify differentially expressed genes between non-invasive and invasive endometrial lesions in Pten/Mlh1-/- mice with Affymetrix oligonucleotides microarrays. 2. To ascertain if selected candidate genes found to be differentially expressed in complex atypical hyperplasia and endometrioid carcinoma in the mouse model (Aim 1) are useful markers of invasive disease in humans. 3. To determine the effect of exogenous estrogen and progestational compounds on endometrial tumorigenesis in Pten mice using light microscopy, immunohistochemistry and molecular techniques. 4. To evaluate the role of the estrogen receptor alpha on endometrial tumorigenesis in Pten +/- mice.